The present invention relates to a multiple semiconductor laser structure. The structure contains a plurality of laser pn junctions stacked vertically one on top of another and each has an active, light-emitting zone. The laser pn junctions each have an n region and a p region, n+p+ tunnel junctions are disposed between and separate vertically neighboring ones of the laser pn junctions. The n+p+ tunnel junctions have an n+-doped layer and a p+-doped layer, the n+-doped layer adjoins the n region of the laser pn junctions and the p+-doped layer adjoins the p region of another one of the laser pn junctions. A n+-doping concentration and a p+-doping concentration are chosen such that a relatively low electrical resistance of the n+p+ tunnel junctions is obtained during operation. A first contact metallization is disposed on the p region of one of the laser pn junctions, and a second contact metallization is disposed on the n region of another one of laser pn junctions.
In the course of the development of semiconductor lasers of high output power, semiconductor laser structures in which a plurality of semiconductor lasers are coupled to one another in a planar manner and connected in series between the poles of a voltage source have also been produced. The first components of this type were produced by a plurality of semiconductor lasers being soldered to one another in a planar manner. However, it has long been known (see the article by Van Der Ziel, et al., xe2x80x9cAppl. Phys. Lett.xe2x80x9d 41, p. 500, 1982) to connect a plurality of GaAs double-heterostructure lasers to one another by interconnecting in each case two vertically neighboring laser pn junctions by a highly doped n+p+ tunnel junction. The n+p+ tunnel junction contains two thin n- and p-doped semiconductor layers, the n+-doped layer directly adjoining the n region of the one pn junction and the p+-doped layer directly adjoining the p region of the other pn junction. The semiconductor material of the tunnel junction can generally be different from or identical to the semiconductor material of the adjoining pn junctions. If a voltage is applied in a forward direction to the series-connected pn junctions via externally applied contact metallizations, the n+p+ junctions are reverse-biased. On account of the high doping, however, a tunnel junction that has no blocking effect is formed. Rather, in the n+p+ tunnel junction a hole current in the p region is converted into an electron current in the n region. As a result, a highly conductive, virtually metallic contact junction is established between the vertically neighboring pn junctions. It is required for this purpose that the doping concentrations in the layers of the n+p+ tunnel junction lie in the range of 1019 cmxe2x88x923 or above.
The optical fields of the laser structures lying one on top of the other may optionally be coupled with one another or decoupled from one another. This is achieved by the dimensioning of the layer composition and thicknesses.
In the publication by Garcia, et al. in xe2x80x9cAppl. Phys. Lett.xe2x80x9d 71, p. 3752, 1997, there is a description of an InGaAs/AlGaAs laser structure emitting at different wavelengths, in which pn junctions are stacked one on top of the other by epitaxial growth and are separated from one another by a low-impedance n+p+ tunnel junction, in which the n doping is formed by carbon and the p doping is formed by sulfur.
On account of the finite thermal conductivity of the individual layers, however, the following problem arises. In each of the laser pn junctions, heating takes place during operation due to the power loss of the electric current and due to non-radiating recombination processes in the active zones. Since, however, the semiconductor lasers are stacked vertically one on top of the other and are mounted with a common substrate on a heat sink, the heat produced in such a way flows away to the heat sink at different rates from the individual semiconductor lasers. While the semiconductor laser lying closest to the heat sink can dissipate its excess heat relatively quickly to the heat sink, the semiconductor laser at the greatest distance from the heat sink exhibits the greatest steady-state heating during operation, since its excess amount of heat must flow through the entire layer structure to reach the heat sink. If the active zones of the individual lasers of the multiple semiconductor laser structure are configured in the same manner, these lasers will, however, emit light radiation at different wavelengths because of the temperature dependence of the band gap of the semiconductor material used. This is disadvantageous for a number of applications. In particular, the pumping of solid-state lasers requires a narrow-band emission of the semiconductor laser in order to use the absorption bands of the solid-state laser effectively.
It is accordingly an object of the invention to provide a multiple semiconductor laser structure with narrow wavelength distribution that overcomes the above-mentioned disadvantages of the prior art devices of this general type, which is capable of emitting light radiation with a narrow wavelength distribution.
With the foregoing and other objects in view there is provided, in accordance with the invention, a multiple semiconductor laser structure. The structure contains a plurality of laser pn junctions stacked vertically one on top of another and each has an active, light-emitting zone. The be laser pn junctions each have an n region and a p region. The active, light-emitting zones of the laser pn junctions each have a material composition and a given thickness. At least one of the material composition and the given thickness is set to match one another in such a way that an influence of different operating temperatures on an emission wavelength is compensated for during operation. A n+p+ tunnel junction is disposed between and separates each pair of vertically neighboring ones of the laser pn junctions. The n+p+ tunnel junction has an n+-doped layer and a p+-doped layer, the n+-doped layer adjoins the n region of one of the laser pn junctions and the p+-doped layer adjoins the p region of another one of the laser pn junctions. A n+-doping concentration and a p+-doping concentration of the n+-doped layer and the p+-doped, respectively, is chosen such that a relatively low electrical resistance of the n+p+ tunnel junction is obtained during operation. A first contact metallization is disposed on the p region of one of the laser pn junctions, and a second contact metallization is disposed on the n region of another of the laser pn junctions.
In a first embodiment of the present invention, the active zones are respectively formed by the well layers of single or multiple quantum well structures and the thicknesses of the well layers of different pn junctions are chosen differently such that the influence of the different operating temperatures of the pn junctions on their emission wavelengths is compensated during operation. In a practical exemplary embodiment of this, in the case of a semiconductor laser for which a relatively high operating temperature is assumed during operation, a relatively small thickness of the quantum well layer is chosen in order to counteract the temperature-induced band edge lowering.
Instead of quantum well structures, quantum wire structures or quantum dot structures may also be used, the thickness of the quantum wire or of the quantum dot then being chosen in a correspondingly compensatory way.
In a second embodiment of the present invention, the active zones are respectively formed by bulk material layers, the material compositions of different pn junctions of which are chosen such that the influence of the different operating temperatures of the pn junctions on their emission wavelengths is compensated for during operation. A temperature-induced band edge lowering can be counteracted here, for example, by an increased Al content in the AlGaAs bulk material of the pn junction.
A third embodiment contains a combination of the aforementioned first and second embodiments. In this case, both the thicknesses and the material compositions of the active zones are varied in order to compensate for the temperature influence on the band edge lowering.
In accordance with an added feature of the invention, the multiple semiconductor laser structure is to be mounted on a heat sink, and a thickness of the well layers decreases with an increasing distance of a respective one of the pn junctions from the heat sink.
In accordance with a further feature of the invention, the multiple semiconductor laser structure is to be mounted on a heat sink, and a material composition of the bulk material layers is chosen such that an increase in a band gap of the material composition of the bulk material layers is obtained with an increasing distance from the heat sink.
In accordance with another feature of the invention, the n+p+ tunnel junction is one of a plurality of n+p+ tunnel junctions disposed between and separating each pair of the vertically neighboring ones of the laser pn junctions.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a multiple semiconductor laser structure with narrow wavelength distribution, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.